1. Field of the Invention
This invention relates to the production of olefins from hydrocarbon feedstock. More particularly, the invention relates to the production of olefins from heavy hydrocarbon feedstocks. Most specifically, the invention relates to the production of olefins from heavy hydrocarbon feedstocks by a combination of pretreatment of the heavy hydrocarbon feedstock in which a liquid fuel product first is produced as a method of preferentially rejecting carbon to enhance the production of olefins ultimately converted from the hydrocarbon feedstock.
2. Description of the Prior Art
The petrochemical industry has long used naturally forming hydrocarbon feedstocks for the production of valuable olefinic materials, such as ethylene and propylene. Ideally, commercial operations have been carried out using normally gaseous hydrocarbons such as ethane and propane as the feedstock. As the lighter hydrocarbons have been consumed and the availability of the lighter hydrocarbons has decreased, the industry has been required to crack heavier hydrocarbons. Hydrocarbons such as naphtha and atmospheric gas oil (AGO) which have higher boiling points than the gaseous hydrocarbons have been used commercially. Processes are under development for the use of still heavier, less expensive feeds such as vacuum gas oil (VGO) or residues from atmospheric distillation columns, commonly called atmospheric tower bottoms (ATB). However, none of these processes using feeds with normal boiling points above 650.degree. F. have achieved broad commercial success. The major impediment to the use of feeds heavier than AGO has been the need to increase the quantity of dilution steam necessary to inhibit coke formation. A further problem results from the need to dispose of increasing yields of the poor quality fuel oil by-product of the olefin producing process when the heavy hydrocarbons are used as feedstock.
A typical process for the production of olefins from naturally forming hydrocarbon feedstocks is the thermal cracking process.
Illustratively, process fired heaters are used to provide the requisite heat for the reaction. The feedstock flows through a plurality of coils within the fired heater, the coils being arranged in a manner that maximizes the heat transfer to the hydrocarbon flowing through the coils. In conventional coil pyrolysis, dilution steam is used to inhibit coke formation in the cracking coil. A further benefit of high steam dilution is the inhibition of the coke deposition in the exchangers used to rapidly quench the cracking reaction. An illustration of the conventional process is seen in U.S. Pat. No. 3,487,121 (Hallee). More recently, the thermal cracking process has been conducted in apparatus which allow the hydrocarbon feedstock to pass through a reactor in the presence of steam while providing for heated solids as the heat carrier.
The use of steam in the hydrocarbon stream requires larger furnace capacity and equipment than would be necessary for the hydrocarbon without steam. Further, when steam is used, energy and equipment must be provided to generate and superheat the steam.
In the production of olefins from hydrocarbon feedstocks the generation of coke has been a problem regardless of the process used. Typically, the cracking reaction will cause production of heavy tar and coke materials which foul the equipment and provide no useful function. The problem is particularly acute in the coil cracking environment where the furnaces must be taken from service to remove the coke and tar from the coils to enable the process to continue efficiently.
The use of heavier hydrocarbon feedstocks, such as residual oils with the attendant high asphaltene and coke precursor content, intensifies and magnifies the problem of coke formation and the associated equipment fouling problems in coil processes. To compensate, the steam rate must be increased, which would increase the specific energy, i.e., energy consumed per unit of ethylene/olefins produced. When using VGO as a feedstock, for example, the specific energy can be 50% above that needed for a light hydrocarbon such as naphtha. Similarly, the amount of ethylene that can be produced from a given size pyrolysis coil when using VGO is often less than half that obtained from naphtha.
Another problem attendant to the use of higher boiling range feedstocks is the increased production of poorer quality fuel oil. The cracking severity needed to produce olefins from these heavy feeds is much higher than that used for conventional thermal crackers designed to produce gasoline and fuel oil. This high severity results in the simultaneous production of olefins and poor quality fuel oil rich in asphaltenes and free carbon.
All of the above problems have detracted from the use of high boiling, less expensive feeds for producing olefins.
A variety of attempts have been made to pretreat the heavy hydrocarbon feedstock to render it suitable for thermal cracking. Hydro-treating of the feedstock is one effort. Another effort, is the vaporization of the feedstock with large quantities of steam to create a very low system partial pressure (Gartside, U.S. Pat. No. 4,264,432). Others have proposed solvent extraction pretreatment of the hydrocarbon to remove the asphaltene and coke precursors. Another attempt is the thermal pretreatment of resids to yield a heavy hydrocarbon, then catalytically hydrotreating a portion of the heavy hydrocarbon feedstock before the steam cracking step (U.S. Pat. No. 4,065,379, Soonawala, et al.) and similarly, the pre-treatment of hydrocarbon feedstock by initial catalytic cracking to produce a naphtha or naphtha-like feed for ultimate thermal cracking (U.S. Pat. No. 3,862,898, Boyd, et al.). These processes all improve the cracking of heavy hydrocarbon, however, in most instances the process suffers from either the expense of large steam dilution equipment or the unsatisfactory reduction of tar and coke accumulation in the process equipment.